HELGOLA.NDER MEERESUNTERSUCHUNGEN Helgol6nder Meeresunters. 52, 347-357 (1999)
The recent arrival of the oceanic isopod I d o t e a m e t a l l i c a Bosc off Helgoland
(German Bight, North Sea):
an indication of a w a r m i n g trend in the North Sea?
H.-D. F r a n k e , L. G u t o w & M. J a n k e
Biologische Anstalt Helgoland, Stiftung A l f r e d - W e g e n e r - l n s t i t u t fiJr Polar- und NIeeresforschung, Meeresstation Helgoland, Postfach 180,
D-27483 Helgoland, G e r m a n y
ABSTRACT: In 1988 a long-term study was started of the isopod fauna associated with surface drift material off Helgoland (German Bight, North Sea}. In the summer of 1994 s p e c i m e n s of ldoteu metullica Bosc were recorded for the first time. There is no evidence that this species has ever b e e n p r e s e n t in the G e r m a n Bight before. The s a m p l e s contained males, both gravid and non-gravid fe- males, and juveniles, indicating that the species r e p r o d u c e d successfully in the Helgoland region.
Interbreeding of s p e c i m e n s from Helgoland and the western Mediterranean p r o d u c e d fertile off- spring. As a neustonic species, 1. metallicu shows a high natural capacity for dispersal. It thus s e e m s unlikely that the arrival of the species in the North Sea resulted from an accidental intro- duction by man. We are probably witnessing an extension of the species' geographical range by natural m e a n s of dispersal, as a response to recent c h a n g e s in the ecological conditions of the Ger- man Bight. Temperature data m e a s u r e d by the Biologische Anstalt Helgoland since 1962 show that the last d e c a d e (except 1996) was characterized by unusually mild winters. Following the severe winter of 1996, I. metallica was again a b s e n t from the Helgoland region. After the s u b s e q u e n t mild winters (1997 and 1998), however, the species r e a p p e a r e d in the summer of 1998 with higher num- bers than ever before. This suggests that the observed p h e n o m e n a are closely c o n n e c t e d with the recent t e m p e r a t u r e anomalies. I. metallica can be r e g a r d e d as a potential immigrant to a warmer North Sea, and may be useful as a sensitive indicator of the predicted long-term warming trend.
I N T R O D U C T I O N
C h a n g e s in t h e g e o g r a p h i c r a n g e s of s p e c i e s a n d t h u s t h e a p p e a r a n c e of s p e c i e s in, a n d t h e i r d i s a p p e a r a n c e f r o m , p a r t i c u l a r a r e a s , is a p h e n o m e n o n as o l d a s life itself, d r i - v e n b y g r a d u a l e v o l u t i o n a r y c h a n g e s of t h e s p e c i e s , i n t e r a c t i n g w i t h e n v i r o n m e n t a l c h a n g e s s u c h a s t h o s e i n c l i m a t e . It is o n l y w i t h t h e r a p i d l y i n c r e a s i n g i m p a c t of h u m a n s o n t h e e a r t h ' s e c o l o g y , h o w e v e r , t h a t t h e p a c e of c h a n g e in t h e d i s t r i b u t i o n p a t t e r n s of s p e c i e s h a s i n c r e a s e d d r a m a t i c a l l y ( H e n g e v e l d , 1990).
In g e n e r a l , t h e a r r i v a l of a s p e c i e s a t a l o c a l i t y w h e r e it h a s n o t b e e n f o u n d b e f o r e c a n r e s u l t f r o m t h e f o l l o w i n g p r o c e s s e s :
t. T h e s p e c i e s h a s u n d e r g o n e s o m e e v o l u t i o n a r y c h a n g e in its g e n e t i c m a k e - u p , e n - a b l i n g it to e x t e n d its r a n g e to l o c a l i t i e s w h e r e it h a s n o t b e e n a b l e to e x i s t b e f o r e . 9 Biologische Anstalt Helgoland, H a m b u r g
348 H.-D. Franke, L. G u t o w & M. J a n k e
2. T h e ecological conditions in a particular locality h a v e c h a n g e d , w h e t h e r for n a t u r a l r e a s o n s or as a result of h u m a n activities, in such a way that a species will experi- e n c e f a v o u r a b l e living conditions in places w h e r e this has not b e e n the case before.
3. T h e species (often assisted a c c i d e n t a l l y or i n t e n t i o n a l l y by man) has s u c c e e d e d in o v e r c o m i n g barriers to dispersal which h a v e p r e v e n t e d it in the past from g a i n i n g access to a n d c o l o n i z i n g other s u i t a b l e localities.
Surface drift m a t e r i a l (mainly uprooted m a c r o a l g a e ) r e p r e s e n t s a specific n e u s t o n i c h a b i t a t which provides a c o n s i d e r a b l e n u m b e r of m a c r o f a u n a l species with a s u b s t r a t e to cling to, shelter from pelagic predators, a n d / o r food. A l t h o u g h a c c u m u l a t i o n s of drift m a t e r i a l are c o m m o n in m a n y coastal regions, they h a v e not yet attracted m u c h a t t e n - tion in ecological research (Tully & C6idigh, 1986; Locke & Corey, 1989). Isopods are a significant m a c r o f a u n a l e l e m e n t i n h a b i t i n g drift s e a w e e d . D u r i n g a l o n g - t e r m study of the isopod f a u n a associated with drift s e a w e e d , we recorded i n d i v i d u a l s of a species, l d o t e a m e t a l l i c a Bosc, which has n e v e r b e e n recorded before in this area.
T h e g e o g r a p h i c a l distribution of I. m e t a l l i c a was d e s c r i b e d by Naylor (1957). Resi- d e n t b r e e d i n g localities are the east coast of North America (from Florida to Nova Sco- tia) as well as the M e d i t e r r a n e a n a n d Black Seas (Dow & Menzies, 1958; Abell6 & Fran- k l a n d , 1997). F u r t h e r m o r e , there are occasional records from m a n y other parts of the world, from tropic, subtropic, t e m p e r a t e a n d e v e n cold waters. However, it is impossible to d e c i d e from the literature data w h e t h e r these records indicate the existence of further r e s i d e n t b r e e d i n g populations. I. metallicu is principally a surface dweller, associated with drift material such as w e e d a n d timber. It is this habit which confers a high p o t e n - tial for dispersal on the species a n d p r o b a b l y a c c o u n t s for the species' n e a r l y cos- m o p o l i t a n distribution.
In the p r e s e n t p a p e r we report on our records of I. m e t a l l i c a off H e l g o l a n d from the s u m m e r of 1994 u p to n o w ( s u m m e r 1998). Crossing e x p e r i m e n t s were p e r f o r m e d to s t u d y w h e t h e r North Sea a n d M e d i t e r r a n e a n s p e c i m e n s of I. m e t a l l i c a really b e l o n g to the s a m e species. Finally, data on water t e m p e r a t u r e s at H e l g o l a n d were a n a l y s e d to look for possible causes of the o b s e r v e d p h e n o m e n a .
MATERIALS AND M E T H O D S
T h e h y d r o g r a p h i c situation a n d the t o p o g r a p h y of the H e l g o l a n d r e g i o n give rise to the formation of characteristic b o u n d a r y zones b e t w e e n tidal currents of different s p e e d a n d / o r direction. T h e s e zones d e v e l o p both south a n d north of the island. A l o n g these zones, surface drift material a c c u m u l a t e s . This is particularly e v i d e n t d u r i n g periods of c a l m weather, i.e. particularly in the summer. T h e a c c u m u l a t i o n of drift m a t e r i a l occurs m a i n l y south of the i s l a n d w h e n the tide rises, a n d n o r t h d u r i n g the falling of the tide.
T h e a c c u m u l a t i n g m a t e r i a l is composed m a i n l y of u p r o o t e d m a c r o a l g a e such as F u c u s sp., A s c o p h y I l u m n o d o s u m , and, to a lesser extent, S u r g a s s u m m u t i c u r n a n d H i m a n - thalia e l o n g a t a . M u c h of the m a c r o a l g a l debris f o u n d off H e l g o l a n d p r o b a b l y does not o r i g i n a t e from the rocky i s l a n d itself b u t is carried a l o n g by w a t e r currents from coastal r e g i o n s of Britain a n d the W a d d e n Sea area. Timber, plastic waste, cables a n d r e m a i n s of fishing nets also occur f r e q u e n t l y a l o n g the b o u n d a r y zones.
T h e recent arrival of the oceanic isopod Idotea rnetallica Bosc 349
In 1988 we started a (semio)quantitative l o n g - t e r m s a m p l i n g p r o g r a m m e of the Idotea species associated with surface drift m a t e r i a l off H e l g o l a n d . Every s u m m e r (July to October), a n u m b e r of s a m p l e s were t a k e n at i r r e g u l a r intervals d e p e n d i n g on w e a t h e r conditions. Drift material was collected, from a b o a r d a motor boat, u s i n g a h a n d n e t of 1-mm m e s h size. A total of 99 s a m p l e s were collected d u r i n g the 11oyear period from 1988 to 1998. T h e s a m p l e size varied b e t w e e n a b o u t 1 a n d 20 kg of algal material. T h e a n i m a l s were s e p a r a t e d q u a n t i t a t i v e l y from the a l g a e by carefully wash- i n g the m a t e r i a l with seawater.
T h e s a m p l e s u s u a l l y c o n t a i n e d a great n u m b e r (often several thousands) of m a n c a s a n d y o u n g j u v e n i l e s . However, only s p e c i m e n s with a body l e n g t h of more t h a n 10 m m ( a d v a n c e d j u v e n i l e s a n d adults) were c o n s i d e r e d for the study b e c a u s e it is only this fraction which can be identified easily to species level. A b u n d a n c e s were e x p r e s s e d in n u m e r i c a l densities ( n u m b e r of i n d i v i d u a l s p e r kilogram fresh w e i g h t of algae). Addi- tionally, in the s u m m e r of 1995, a total of a b o u t 1200 s a m p l e d l d o t e a s p e c i m e n s with a b o d y l e n g t h of less t h a n 10 m m (mancas: 3-6 ram; y o u n g juveniles: 6-10 mm) were raised in the laboratory until they could be d e t e r m i n e d easily to species level. O n e h u n - dred i n d i v i d u a l s each were kept in plastic cups with 5 1 of a e r a t e d seawater. T h e water was c h a n g e d every 3-4 days. Food (fragments of the b r o w n alga A s c o p h y l l u m n o d o s u m a n d freshly h a t c h e d A r t e m i a nauplii) was a v a i l a b l e ad libitum.
During a cruise of the Biologische Anstalt H e l g o l a n d (BAH) research vessel
" H e i n c k e " in S e p t e m b e r 1995 from Lisbon (Portugal) via G i b r a l t a r to Nice (France), surface drifting material was collected in the w e s t e r n M e d i t e r r a n e a n a n d was e x a m i n e d with respect to associated isopods. Some h u n d r e d s of I. m e t a l l i c a were isolated from this material a n d were t a k e n to H e l g o l a n d by p l a n e in order to be u s e d in crossing ex- p e r i m e n t s with North Sea s p e c i m e n s . Reciprocal crossings (North Sea males • Mediter- r a n e a n females, a n d vice versa) were performed. T h e F1 g e n e r a t i o n was tested for fer- tility by crossing a m o n g each other.
Since 1962, on every w o r k d a y surface w a t e r t e m p e r a t u r e s at H e l g o l a n d have b e e n m e a s u r e d a l o n g with other h y d r o g r a p h i c p a r a m e t e r s by the BAH. All calculations of possible c h a n g e s in t e m p e r a t u r e s d u r i n g the 37oyear period from 1962 to 1998 are b a s e d on this time series.
RESULTS
Surface drift material was f o u n d to attract a great variety of m a c r o f a u n a l species such as fish larvae, d e c a p o d larvae, isopods, a m p h i p o d s a n d other small crustaceans.
T h e h a b i t a t was d o m i n a t e d q u a n t i t a t i v e l y b y different species of the isopod g e n u s Idotea a n d by the a m p h i p o d G a m m a r u s locusta.
All eight species of I d o t e a k n o w n from British waters (Naylor, 1972) w e r e also f o u n d a m o n g drift s e a w e e d off H e l g o l a n d (Table 1). T h e p r i n c i p a l isopod i n h a b i t a n t of drift material was l d o t e a baltica. It was the only species of the g e n u s r e p r e s e n t e d in all 99 samples; furthermore, it was always by far the most a b u n d a n t of all I d o t e a species, r e p r e s e n t i n g 63.9-98.4% (mean: 89.1 +- 7.7%) of the total n u m b e r s of i n d i v i d u a l s of a sample. I. pelagica, I. granulosa, I. chelipes, I. e m a r g i n a t a , I. linearis a n d I. n e g l e c t a do not typically i n h a b i t drift material. T h e s e species h a v e d i v e r g e n t specific habitats, yet
350 H.-D. F r a n k e , L. G u t o w & M. J a n k e
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T h e r e c e n t a r r i v a l of t h e o c e a n i c i s o p o d Idotea metallica B o s c 351
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352 H.-D. F r a n k e , L. G u t o w & M. J a n k e
Fig. 1. Annual course of monthly means of surface seawater temperature at Helgoland for the 26-year period 1962-1987 (O) and the recent 11-year period 1988-1998 (0)
m a y associate occasionally with drift material, u s i n g it as a m e a n s of p a s s i v e dispersal (Naylor, 1955; p e r s o n a l observations). In contrast, I. b a l t i c a a n d I. m e t a l l i c a s e e m to live p r e d o m i n a n t l y a m o n g s t drift s e a w e e d which r e p r e s e n t s the species' typical h a b i t a t in North Atlantic waters (Naylor, 1955, 1957; H o l d w a y & Maddock, 1983; Locke & Corey, 1989).
In the s u m m e r of 1994 we recorded, for the first time, a few s p e c i m e n s of I. m e t a l - lica, i n c l u d i n g o n e gravid (ovigerous) female. T h e s p e c i m e n s conformed to the descrip- tion g i v e n by Naylor (1957, 1972). In the s u m m e r of 1995, I. m e t a l l i c a w a s a g a i n p r e s e n t in the H e l g o l a n d region. T h e n u m b e r of i n d i v i d u a l s (per kilogram fresh w e i g h t of col- lected s e a w e e d ) was slightly h i g h e r t h a n in the previous year. A m o n g the 32 s p e c i m e n s collected were 6 gravid females. This material was u s e d to establish a l a b o r a t o r y mass culture of I. m e t a l l i c a (details will b e reported elsewhere). Because n o s a m p l e s w e r e collected d u r i n g the w i n t e r of 1994/1995, it is not clear w h e t h e r the species really over- w i n t e r e d in the G e r m a n Bight or w h e t h e r it did not survive the w i n t e r c o n d i t i o n s a n d t h e n r e t u r n e d the following year, its p r e s e n c e in the s u m m e r of 1994 a n d 1995 thus rep- r e s e n t i n g two i n d e p e n d e n t p h a s e s of immigration.
After the s e v e r e w i n t e r of 1995/1996, the species was c o m p l e t e l y a b s e n t from the s a m p l e s collected in s u m m e r 1996. In the s u m m e r of 1997, only five s a m p l e s w e r e t a k e n , a n d a g a i n no s p e c i m e n s of I. m e t a l l i c a were found. In the s u m m e r 1998, how- ever, the species r e a p p e a r e d , there n o w b e i n g h i g h e r n u m b e r s t h a n e v e r before, indi- c a t i n g that its a p p e a r e n c e in 1994 a n d 1995 was not a u n i q u e event.
T h e r e c e n t arrival of the oceanic isopod ldotea m e t a l l i c a Bosc 353
Fig. 2. Lowest monthly means of surface seawater temperature at Helgoland for years 1988-1998, expressed as deviation (in ~ from the long-term average (period 1962-1987) which has been set
equal to zero
M a n c a s a n d y o u n g j u v e n i l e s collected from drift material in the s u m m e r of 1995 were reared successfully in the laboratory. After 6 weeks, the surviving i n d i v i d u a l s (mortality rate b e t w e e n 10~ a n d 36%, m e a n : 20.2 • 8.1'7,,) could be d e t e r m i n e d easily to species level. In addition to the bulk of i n d i v i d u a l s which t u r n e d out to be I. baltica, we recorded i n d i v i d u a l s of some other l d o t e a species, i n c l u d i n g a small n u m b e r of I. m e t a l l i c a (Table 2). This indicates that all stages of I. m e t a l l i c a were p r e s e n t in the samples, a n d that the species is able to r e p r o d u c e successfully in the H e l g o l a n d region.
O n drift m a t e r i a l collected in the w e s t e r n M e d i t e r r a n e a n , L m e t a l l i c a was the prin- cipal (and u s u a l l y the only) isopod i n h a b i t a n t (details will be reported elsewhere). All d e v e l o p m e n t a l stages of L m e t a l l i c a w e r e r e p r e s e n t e d in the samples, a g a i n i n d i c a t i n g that the species u n d e r g o e s its complete life cycle i n association with surface drift m a t e - rial ( e u n e u s t o n i c species). I. baltica was the only other I d o t e a species which was found, in small n u m b e r s , a m o n g drift m a t e r i a l in the M e d i t e r r a n e a n . Crossings b e t w e e n spec- i m e n s from the M e d i t e r r a n e a n a n d the North Sea were successful, i n d e p e n d e n t of the origin of the m a l e a n d female p a r t n e r (reciprocal crossings). T h e F1 g e n e r a t i o n p r o v e d to be fertile a n d did not show a n y obvious difference in fitness c o m p a r e d to the P g e n - eration.
Frem the n o w available 37-year data series of w a t e r t e m p e r a t u r e s at H e l g o l a n d (an- n u a l cycles of m o n t h l y means), we c a l c u l a t e d s e p a r a t e a v e r a g e curves for the 26~
period 1962-1987 a n d the l l - y e a r period 1988-1998. M o n t h l y m e a n s for the period
354 H.-D. F r a n k e , L. G u t o w & M. J a n k e
1988-1998 p r o v e d to be consistently h i g h e r over the complete a n n u a l cycle c o m p a r e d to the period 1962-1987 (Fig. 1). T h e difference is particularly e v i d e n t for the w i n t e r months, less obvious in s p r i n g a n d summer, a n d least a p p a r e n t in a u t u m n . T h e g e n e r a l w a r m i n g , i.e. the i n c r e a s e in the a n n u a l m e a n t e m p e r a t u r e ( 1 9 6 2 - 1 9 8 7 : 9 . 6 ~ 1988- 1998:10.3 ~ was thus m a i n l y d u e to a n i n c r e a s e in w i n t e r temperatures. In 10 out of the 11 years of the r e c e n t period 1988-1998, the lowest m o n t h l y m e a n t e m p e r a t u r e (usually in February, occasionally in J a n u a r y a n d March, respectively) was a b o v e the respective 26-year (1962-1987) l o n g - t e r m a v e r a g e of 2.7 ~ (Fig. 2). The only e x c e p t i o n w a s the severe w i n t e r of 1996 with a lowest m o n t h l y m e a n (February) of 0.7 ~
DISCUSSION
From n o r t h - w e s t E u r o p e a n waters, there h a v e b e e n occasional records of I. m e t a l - l i c a off the west coast of the British Isles. Naylor (1957) s u g g e s t e d that the species is t r a n s p o r t e d across the Atlantic from North America, occasionally r e a c h i n g British wa- ters a m o n g s t floating objects, carried a l o n g by the North Atlantic Drift. A l t h o u g h the species has occasionally b e e n recorded off the west coast of Britain for the past 150 years or so, it "has not so far b e e n able to establish itself as a British r e s i d e n t " (Naylor, 1957). T h e r e is no i n d i c a t i o n of w h e t h e r this situation has c h a n g e d since. More recent records from Irish waters have b e e n g i v e n by M c G r a t h (1980) a n d Tully & McGrath (1987).
As to the North Sea, only two c h a n c e finds of a single s p e c i m e n each h a v e b e e n re- ported for the N o r w e g i a n a n d the Dutch coast, respectively {Pethon, 1970; H u i j s m a n &
H u w a e , 1978). As far as we know, the species has n e v e r b e e n recorded in the G e r m a n Bight before our first records in the s u m m e r of 1994. This is u n l i k e l y to s i m p l y reflect a lack of diligent e x a m i n a t i o n of s a m p l i n g s : the G e r m a n Bight (and particularly the Hel- g o l a n d region) has b e e n a n e x t e n s i v e l y s t u d i e d m a r i n e area for more t h a n a h u n d r e d years, a n d I. m e t a l l i c a is a very c o n s p i c u o u s species with respect to h a b i t u s a n d colour.
So, if significant n u m b e r s of the species w e r e p r e s e n t off H e l g o l a n d before 1994, they s h o u l d certainly h a v e b e e n noticed. Particularly in the period 1988-1993, d u r i n g the first part of our study, we can definitely e x c l u d e the p r e s e n c e of I. m e t a l l i c a off Hel- g o l a n d : a m o n g m a n y t h o u s a n d s of I d o t e a i n d i v i d u a l s which were carefully e x a m i n e d , there was not a single s p e c i m e n of I. m e t a l l i c a .
T h e origin of the North Sea s p e c i m e n s of I. m e t a l l i c a r e m a i n s open. In our crossing e x p e r i m e n t s s p e c i m e n s from the M e d i t e r r a n e a n a n d the North Sea were f o u n d to be ca- p a b l e of i n t e r b r e e d i n g , a n d m a y thus be r e c o g n i z e d as m e m b e r s of the s a m e species.
Nevertheless, r e g a r d i n g the h y d r o g r a p h i c p a t t e r n of the North Atlantic, it s e e m s to be more likely that the North Sea s p e c i m e n s o r i g i n a t e from North A m e r i c a a n d the North Atlantic rather t h a n from the M e d i t e r r a n e a n . O n l y a g e n e t i c analysis c a n p r o v i d e con- clusive evidence.
C o n s i d e r i n g the species' high n a t u r a l capacity for dispersal, it a p p e a r s very u n - likely that the arrival of I. m e t a l l i c a in the North Sea resulted from a n a c c i d e n t a l intro- d u c t i o n by man. Most likely, we are w i t n e s s i n g a n e x p a n s i o n of the r a n g e of the species into the North Sea by its n a t u r a l m e a n s of dispersal, as a r e s p o n s e to c h a n g i n g e n v i r o n - m e n t a l conditions. It is h a r d l y a simple c o i n c i d e n c e that the first records of I. m e t a l l i c a
T h e r e c e n t arrival of the oceanic isopod Idotea m e t a l l i c a Bosc 355
off H e l g o l a n d occurred d u r i n g a s u m m e r (1994) that was p r e c e d e d by a period of mild winters (1989-1994), a n d that the species was a g a i n a b s e n t from the area after the se- vere w i n t e r of 1996. T h e species' r e a p p e a r a n c e in the s u m m e r of 1998 was a g a i n pre- c e d e d by two mild winters. This suggests that some feature related to t e m p e r a t u r e , p r o b a b l y a lack of e x t r e m e l y low w a t e r t e m p e r a t u r e in winter, is r e l e v a n t to the ob- served p h e n o m e n o n .
T h e r e is a g r o w i n g conviction that global t e m p e r a t u r e s (including sea surface tem- peratures) are rising, a n d that this g e n e r a l w a r m i n g t r e n d results for the most part from m a n ' s activities (Folland et al., 1992; Mitchell et al., 1995; IPCC, 1996). Because of the complexity of the p h e n o m e n a , however, there is still c o n s i d e r a b l e u n c e r t a i n t y as to the details of the a n t i c i p a t e d climate c h a n g e (extent a n d s p e e d of c h a n g e , differences on regional scales).
Parameters c o n n e c t e d with e n v i r o n m e n t a l t e m p e r a t u r e are a m o n g the most impor- tant physical v a r i a b l e s controlling the large-scale distributions a n d the a b u n d a n c e s of m a r i n e organisms, t h o u g h it is often not clear w h a t aspects of t e m p e r a t u r e are the cru- cial limiting conditions a n d what their exact mode of action is (Orton, 1920; H e n g e v e l d , 1990; Bhaud et al., 1995). It is not surprising, thus, that in most scenarios the p r e d i c t e d climate c h a n g e will have profound implications for ecological conditions (e.g. Vitousek, 1994). Rather t h a n by m e a n t e m p e r a t u r e s a n d the r a n g e c o m m o n l y e x p e r i e n c e d , distri- b u t i o n s a n d a b u n d a n c e s of m a n y species are strongly i n f l u e n c e d by the extremes of t e m p e r a t u r e . As to the c o n t i n e n t a l l y i n f l u e n c e d North Sea, it is w i n t e r t e m p e r a t u r e that is particularly r e l e v a n t to the composition a n d f u n c t i o n i n g of the e c o s y s t e m (e.g. Beu- kema, 1990).
A recent t e m p e r a t u r e increase in coastal waters of the s o u t h e r n North Sea is evi- dent. Nevertheless, it is p r e s e n t l y still impossible to assess w h e t h e r this is simply a short-term p h e n o m e n o n similar to p h e n o m e n a which have already b e e n recorded oc- casionally in the past, e.g. a r o u n d 1974 , or w h e t h e r we are really w i t n e s s i n g the b e - g i n n i n g of the p r e d i c t e d l o n g - t e r m w a r m i n g trend (Becker & Pauli, 1996). No m a t t e r how, the r e s p o n s e s of p o p u l a t i o n s a n d ecosystems to periods with mild w i n t e r s c a n give us some idea of w h a t m i g h t h a p p e n in a w a r m e r North Sea (Beukema, 1992). T h e r e c a n b e little d o u b t that a persistent w a r m i n g trend, particularly a lack of e x t r e m e l y cold winters, will h a v e serious c o n s e q u e n c e s on the structure a n d f u n c t i o n i n g of the North Sea ecosystem. A lack of severe w i n t e r s will favour the large n u m b e r of North Sea species which are k n o w n to be sensitive to low t e m p e r a t u r e s (Beukema, 1992), a n d species with a more oceanic (southerly) distribution p a t t e r n m a y be e x p e c t e d to e x t e n d their r a n g e s n o r t h e a s t w a r d s into the North Sea (Greve, 1994; G r e v e et al., 1996). Al- t h o u g h there are also species which will p r o b a b l y be n e g a t i v e l y affected by a w a r m i n g t r e n d a n d m i g h t finally e v e n d i s a p p e a r from the area, it has b e e n p r e d i c t e d that species richness in the W a d d e n Sea (and this m a y also a p p l y to the North Sea as a whole) m a y increase rather t h a n decrease in a w a r m e r climate (Beukema, 1990, 1992; de Vooys, 1990).
The o c e a n i c I. m e t a l l i c a m a y be o n e of the p o t e n t i a l i m m i g r a n t s to a w a r m e r North Sea, w h e t h e r as a r e g u l a r s u m m e r i m m i g r a n t or e v e n as a p e r m a n e n t resident. A cli- m a t e shift, however, is only a n e c e s s a r y p r e c o n d i t i o n for the species to e x t e n d its geo- graphical r a n g e into the North Sea. W h e t h e r the species c a n take full a d v a n t a g e of a n altered situation a n d will really establish itself p e r m a n e n t l y in the n e w e n v i r o n m e n t de-
356 H . - D . F r a n k e , L. G u t o w & M. J a n k e
p e n d s o n w h e t h e r it is l i m i t e d p r i m a r i l y b y c l i m a t i c f a c t o r s r a t h e r t h a n b y c o m p e t i t i v e ( a n d / o r o t h e r b i o l o g i c a l ) o n e s . B e c a u s e of t h e c o m p l e x i t y of t h e r e l e v a n t b i o l o g i c a l in- t e r a c t i o n s , t h e e x a c t e c o l o g i c a l c o n s e q u e n c e s of a c l i m a t e s h i f t for a p a r t i c u l a r a r e a a r e h a r d to p r e d i c t .
It is g e n e r a l l y k n o w n t h a t t h e a r r i v a l of a n e w e l e m e n t i n a p a r t i c u l a r e n v i r o n m e n t m a y h a v e a s i g n i f i c a n t , s o m e t i m e s h i g h l y d e t r i m e n t a l , i m p a c t o n a n e s t a b l i s h e d e c o s y s - t e m . D r a m a t i c r e c e n t e x a m p l e s a r e t h e a r r i v a l of t h e a l g a C a u l e r p a t a x i f o l i a in t h e M e d i t e r r a n e a n (e.g. B e l l a n o S a n t i n i e t al., 1996), t h e C t e n o p h o r e M n e m i o p s i l e i d y i in t h e B l a c k S e a (e.g. K i d e y s , 1994), a n d t h e z e b r a m u s s e l D r e i s s e n a p o l y m o r p h a i n t h e N o r t h A m e r i c a n G r e a t L a k e s (e.g. L u d y a n s k i y e t al., 1993), to n a m e b u t a few. In c o n - t r a s t , t h e r e a r e a l s o m a n y c a s e s of n e w l y a r r i v e d s p e c i e s w h i c h w e r e a b l e to f i n d u n e x - p l o i t e d r e s o u r c e s a n d t h u s b e c a m e a s s i m i l a t e d i n t o c o m m u n i t i e s w i t h o u t a n y o b v i o u s i m p a c t o n t h e n a t i v e b i o t a . T h e r e s p o n s e s of I. m e t a l l i c a to d i f f e r e n t t e m p e r a t u r e r e g i m e s as w e l l as t h e e f f e c t s w h i c h a s u c c e s s f u l l y i m m i g r a t i n g I. m e t a l l i c a m i g h t e x e r t o n c o m p e t i n g n a t i v e s p e c i e s ( a n d v i c e v e r s a ) a r e t h e s u b j e c t of c u r r e n t l a b o r a t o r y s t u d - ies. T h e r e is n o i n d i c a t i o n t h a t I. m e t a l l i c a m i g h t b e c o m e a k e y p o p u l a t i o n w i t h a g r e a t i m p a c t o n t h e N o r t h S e a e c o s y s t e m ; h o w e v e r , t h e s p e c i e s m a y s e r v e a s a s e n s i b l e i n d i - c a t o r of a p o s s i b l e w a r m i n g t r e n d in t h e s o u t h e r n N o r t h S e a .
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